How to Install a Firewall

There are many "mothers of invention". While necessity often dictates development, so too have simple mistakes. Take, for example, forgetting to install/facilitate an updated frame-mount brake pedal and master cylinder assembly on a post-war Chevy before having the frame powdercoated and routing the exhaust. No problem, right? Simply incorporate some fashion of a firewall-mounted brake setup. That would be fine and dandy under one circumstance: You're not opposed to having a late-model master/booster hanging off the front of your firewall. In many cases, the answer is "no". OK, how about integrating an underdash setup? That too would be fine and dandy if it weren't for the fact that you'd already smoothed the exterior of the firewall, thus not visually allowing any means in which to attach...at least not a traditional manner, that is.

Hindsight reared mama invention's beautiful head in this particular situation, ultimately producing a panacea for not only the brake system dilemma, but nearly every issue that smoothing a firewall can create when it comes to mounting/attaching items. Sheetmetal pales in comparison to a piece of 3/16-inch aluminum plate—it also tears under any kind of significant load! With a plate panel mounted behind the firewall, your underdash accessorizing is almost unlimited, including the main reason why we're here in the first place: accommodating an above-floor resolution for not having re-incorporating the OE under-floor brake pedal assembly when the opportunity existed.

Jimenez Bros. Customs (JBC) starts off with a sturdy plate foundation cut to shape based off the parameters presented behind the upper-inner firewall (just below the cowl) mounted via captured nuts welded to the body. With such a solid base, a Classic Performance Products beefy swing-pedal setup is able to be attached without fear of any structural failure, even under the constant pressure it will be once the vehicle's on the road and the pedal pumped regularly. Further real estate is acquired for locating such items as the fuse panel, in this case those provided with the Keep It Clean wiring kit the 1947 Chevy in question is using, as well as the battery and master kill switch (also obtained from Keep It Clean).

JBC's inner firewall panels began more as a per-vehicle, custom-made deal. But after having done a number for 1949-54 Chevy cars and 1947-55 trucks (as well as Tri-Fives by the time this comes out), the panels are starting to make their way into the "catalog" category. That said, while JBC would love nothing more than to sell a ton of them to customers far and wide, we're going behind the scenes to show you how they made this one from start to finish! Use the following info to your own discretion.

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3/271. What’s a true custom without a shaved ’n’ smoothed firewall? Well, for one, it’s an area void of allowing any type of through-metal attaching for components beyond its exterior surface—like a wiring panel, or in our case, a brake master cylinder/pedal assembly. Another case of form before function? Let’s see if we can remedy that.

4/272. Sure, welding studs/captured nuts to the interior of the firewall would easily facilitate the mounting of our fuse and relay panels—but for anything of substantial weight, we need to incorporate a substantial device to support not only the weight, but the stress caused by regular use of …

5/273. ...Classic Performance Products’ (CPP) underdash brake kit. One obvious option would be to fabricate a framed structure from square tubing. While that would indeed handle the load, it would also require a lot of real estate below the cowl, which we’d like to avoid, especially if access to the master cylinder (for fluid service) will be required later on.

6/274. Instead, Jimenez Bros. Customs has relied on a more popular solution: an all-inclusive inner firewall panel. As you can see, it all starts with a template—something they’ve saved for vehicles such as ’40s-50s Chevy passenger cars and trucks (meaning they’re available in kit form over the phone).

7/275. A piece of 3/16-inch aluminum plate will serve as the foundation and structural support for all our underdash needs—brake pedal/master, fuse panels, master kill switch, even the battery.

8/276. Since the CPP booster/MC bracket is steel, JBC fashions aluminum tabs that weld to the firewall panel.

9/277. Gussets will be added to further strengthen the mount, helping avoid any structural failure once the brake system is put to use.

10/278. As mentioned, we wanted to retain as much space behind the dash as possible for future access reasons—including having to check/fill the master cylinder. That said, however, we’re also considering a remote-fill option using CPP’s billet-aluminum reservoir lid, tapped for ports allowing fluid to be serviced elsewhere (as long as it’s from a higher vantage point, which is our main obstacle at the moment).

12/2710. The panel is final fitted and points marked where the tabs will be welded onto the car.

13/2711. Once that’s completed, the panel is installed in preparation of component installation. Spanning side to side along the top of the inner firewall, the panel will accept most anything you can dream of, including an A/C unit, which JBC regularly utilizes (the ’47 will not use a hidden system, so instead, we’ll take advantage of the space for our battery).

14/2712. In case we’re unable to enable a remote fill setup, we wanted to see just how close the master cylinder reservoir was to the cowl vent, as that may be a viable option for future access. Of course, one major drawback to that: potential fluid spillage, something a nice paintjob will not accept!

15/2713. Before anything was attached to the panel, we had to incorporate any and all items that could possibly pose interference issues, like the dash with its instrument clusters installed.

16/2714. And to properly locate the brake pedal, we mocked up a dummy steering column using a piece of 1 1/2-inch exhaust tubing, as so.

17/2715. Two sets of hands not only assisted in placement of the assembly prior to welding (including leveling)...

18/2716. but ensured our pedal traveled appropriately (although we’ll likely wind up reshaping the pedal arm later on for both aesthetic and mechanical reasons).

19/2717. After the brake bracket was tack-welded, the panel was removed from the car for final welding and installation of gussets (not shown).

20/2718. Despite the wiring being a chore way down on the “to-do” list, we’re now able to cross off fuse/relay panel installation. Keep It Clean Wiring recently released their Retro Series harness featuring a pre-terminated loom sheathed in a braided housing emulating vintage cloth covering.

21/2719. We had a number of options for where to mount the terminal boxes, but considering the wiring’s already attached, we wanted to do so in a manner that allowed for clean routing of the wires up and out of view.

22/2720. Knowing this also led us to come up with a pedestal-type bracket in which to mount the panels—the flat tabs attach to the firewall plate and are tapped to accept the fuse/relay panels via tube standoffs.

23/2721. The inner firewall panel was once again installed in the vehicle so we could determine the best possible location for wiring components.

24/2722. Once that was done, the holes for the brackets were marked and transferred onto the plate for subsequent drilling and tapping.

25/2723. Here’s how she looked when all (most) was said and done. Notice we rotated the boxes 45 degrees in order to route the wire looms horizontally along the panel, eliminating having to loop them up and around.

26/2724. In the void left where the fuse panel would normally go, the master kill switch will be mounted.

27/2725. Notice we’ve yet to securely attach the XS Power Battery—while weight isn’t a huge concern (the battery mount will gusset off the dash as well), we are looking for a lighter option in the meantime. Beyond that, with our brake pedal situation handled, we can now move onto the plumbing...and any number of items left still "to do"!